Page:The Moon (Pickering).djvu/53

Rh hemispheres with a force which at the mean distance of the two bodies amounts to a tension of 9.6 pounds on the square inch. When the Moon was at one-tenth of its present distance from the Earth this tension would have been one thousand times as great, and would have been sufficient to shatter it to pieces had it then existed in the solid form. If, however, it were fluid or viscous, as we have supposed, the effect would have been merely to produce an enormous tide. In the meantime, if the Moon revolved rapidly on its axis, so that all portions of its surface were presented successively to the Earth, this maximum strain would be felt successively by all portions of its surface, the tendency being to separate or crack it in a meridional direction. We should thus expect to find that the earlier formations would have a tendency to lie in lines in a north-and-south direction. This we find actually to be the case with the craters that we have been discussing, particularly the larger ones. This fact has been pointed out by Webb, Neison and others.

The craters of this early period, of which Copernicus is a characteristic example, would be moulded by the enormous tides into forms resembling Plate A, Figures 4 and 5. The interior surface of one crater, Wargentin, 14B [1.7, 6.2], apparently solidified when the tide which filled it reached to its very rim. The aperture connecting it with the interior had in some way evidently become clogged, and the fluid which had formed the crater was thus caught as it were in the act, to serve as a clue and a perpetual illustration of the process of construction to all future generations. Another crater, Mersenius, 14A [2.5, 4.2], has a conspicuously convex interior. This was the case at first with the paraffine crater represented in Plate A, Figure 4, but subsequent cooling caused it to become concave. If the floors of the lunar craters when they solidified were in general convex, it is evident that the subsequent solidification of the fluid beneath them would tend to make the floor level, thereby producing a compression of the surface, which might well result in the formation of a central peak or ridge. If the paraffine model had been constructed upon a larger scale and the contraction of the fluid beneath it had been allowed to proceed more slowly, it is thought that this result might have been obtained, much as was the case in Figure 2. As it was, a tendency to form small ridges was noticed. In Figure 5 the internal surface of the crater was artificially broken, thereby producing the central mound.

As the cooling process continued, regions deeper down solidified and contracted. The upper layers, having now become completely solid, would not continue to contract